The present invention relates to an adaptive receiving apparatus for a digital communication system and, more particularly, to an adaptive receiving apparatus for land and satellite digital mobile communication systems, which is capable of performing an optimum reception under multi-path fading and low SN ratio conditions.
In a land digital mobile communication system or a satellite digital communication system using BPSK, QPSK or QAM modulation, it is generally necessary to perform an adaptive equalization for multi-path fading distortion in a receiving apparatus. Further, in the case of a mobile communication system in a city area, multi-path distortion due to such factors as reflection off buildings becomes also a subject to be equalized. For the multi-path fading distortion, it has been reported that an adaptive array is effective. For example, Clark, et al. propose utilization of an adaptive array to a land mobile communication as disclosed in "MMSE (Minimum Mean Square Error) Diversity Combining for Wideband Digital Cellular Radio", No. 404.5.1, IEEE Global Telecommunication Conference, 1990. According to this article, it is possible to form a null of an antenna pattern in an incident direction of multi-path waves. By applying such null, a multi-path wave is not received and only main wave is received and, thus, no multi-path distortion occurs. Therefore, it is ideally possible to equivalently remove intersymbol interference due to a multi-path wave by using only an adaptive array, without using a conventional adaptive equalizer having a transversal filter structure.
FIG. 1 shows a typical conventional receiving apparatus having an adaptive array which is disclosed in the above-mentioned article. In FIG. 1, 301 depicts N antennas, 302 N receivers, 303 N multipliers, 304 an adder, 305 a decision device, 306 a subtracter and 307 a LMS (Least Mean Square) operator. The construction shown in FIG. 1 is a well known adaptive array. In order to perform an adaptive control of tap coefficients in the adaptive array, an error signal, which is a difference between an output of the adder 304 and a reference signal, is produced first. As the reference signal, a known training signal or a decision data, which is an output of the decision device 305, is used. The LMS operator 307 updates the tap coefficients of the multipliers 303 by the complex LMS algorithm proposed by Widrow such that mean square value of the error signal becomes minimum. Therefore, the adaptive array shown in FIG. 1 is referred to as an LMS adaptive array and it is known that it operates as follows.
In case where there is no multi-path propagation, the adaptive array makes the antenna pattern to an incident direction of a desired wave so that a receiving signal level becomes maximum. On the other hand, in a case where there is multi-path propagation, a multi-path wave which is in advance or delayed with respect to a main wave (desired wave) become interference. In this case, the adaptive array makes an antenna directivity to the main wave and makes a null in the direction of the multi-path wave propagation. Therefore, it is interpreted that the article of Clark et al. mentioned above utilizes such properties sufficiently. CMA algorithm is also well known as well as LMS algorithm and is being studied. This is to control an envelope level of an output of the adder 304 to a constant level and removes a multi-path wave by means of an adaptive array ultimately.
In the conventional receiving apparatus mentioned above, the multi-path distortion is removed by forming null on an antenna pattern in the direction of the multi-path wave. In such case, when a delay profile due to multi-path propagation is further dispersed, the main wave level is considerably lowered. That is, in the adaptive array method for receiving only the main wave, SN ratio is inherently degraded according to the degree of multi-path dispersion. Therefore, the conventional receiving apparatus using a adaptive array has a drawback that it is impossible to realize an optimum reception in a condition where transmitted power is limited and multi-path propagation arises. Particularly, in a satellite mobile communication system which inherently has low SN ratio condition, a multi-path removing technique capable of improving SN ratio has been required.